I Know, I Know; It's just one of my things, But, Original Lefever bbls were not built on Mono-Blocks. You can't, therefore, have a mono-blocked set of bbls unless you build the block from scratch. You can "Sleeve", "Line", "Re-tube" EtAl but to mono-block you have to have one to start with. This, by definition, means the breech section of the bbls, including lugs was machined from a single block of steel, then bored to accept a pair of tubes.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra